Hypertension

Effects of Garlic on Blood Pressure in Patients With and Without Systolic Hypertension: A Meta-Analysis
Kurt M Reinhart, Craig I Coleman, Colleen Teevan, Payal Vachhani, and C Michael White

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arlic, also known as Allium sativum, has been touted to have cardiovascular benefits that include lowering blood pressure and plasma lipids as well as an antiplatelet effect.1 As such, people with hypertension may choose to use garlic for blood pressure lowering, while others with normal blood pressure may choose garlic for non–blood pressure indications. Over 50 million Americans have hypertension.2 These patients may be classified by hypertension severity; those whose systolic blood pressure (SBP) is greater than 140 mm Hg or whose diastolic blood pressure (DBP) is greater than 90 mm Hg are classified as having stage I hypertension, while those whose SBP is greater than 160 mm Hg or whose DBP is greater than 100 mm Hg are classified as having stage II hypertension.2 Blood pressure reductions in hypertensive patients would be beneficial, given the association between hypertension and cardiac disease and stroke. Antihypertensive therapy decreases the incidence of myocardial infarction, heart failure, and stroke. However, blood pressure reductions could predispose normotensive patients to hypotension.
Author information provided at the end of the text.

BACKGROUND: Garlic has been suggested to lower blood pressure; however, studies evaluating this parameter have provided conflicting results. OBJECTIVE: To examine the effect of garlic on blood pressure in patients with and without elevated systolic blood pressure (SPB) through meta-analyses of randomized controlled trials. METHODS: A systematic search of MEDLINE, CINAHL, and the Cochrane Central Register of Controlled Trials was conducted to identify randomized controlled trials in humans evaluating garlic’s effect on blood pressure. All databases were searched from their inception through June 26, 2008, using the key words garlic, Allium sativum, and allicin. A manual search of published literature was used to identify additional relevant studies. To be included in the analysis, studies must have been written in English or German and reported endpoints of SBP or diastolic blood pressure (DBP). Studies whose population had a mean baseline SBP greater than 140 mm Hg were evaluated separately from those whose population had lower baseline blood pressures. Garlic’s effect on SBP and DBP was treated as a continuous variable and weighted mean differences were calculated using a random-effects model. RESULTS:

Ten trials were included in the analysis; 3 of these had patients with elevated SBP. Garlic reduced SBP by 16.3 mm Hg (95% CI 6.2 to 26.5) and DBP by 9.3 mm Hg (95% CI 5.3 to 13.3) compared with placebo in patients with elevated SBP. However, the use of garlic did not reduce SBP or DBP in patients without elevated SBP. There was only a minor degree of heterogeneity in the analyses and publication bias did not appear to influence the results.

This meta-analysis suggests that garlic is associated with blood pressure reductions in patients with an elevated SBP although not in those without elevated SBP. Future research should focus on the impact of garlic on clinical events and the assessment of the long-term risk of harm.
KEY WORDS:

Ann Pharmacother 2008;42:1766-71.
Published Online, 18 Nov 2008, www.theannals.com, DOI 10.1345/aph.1L319 A For Our Patients summary of this article is available at www.ForOurPatients.info

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England) and a random effects model. which incorporates an assumption that the different studies are estimating different.theannals.5 between initial and final values. A DerSimonian and Laird random-effects model (a variation on the inverse variance method. using the following MeSH terms and key words: garlic. PV.Controlled trials have been conducted in patients with and without elevated SBP or DBP to evaluate the impact of garlic therapy on blood pressure.
DATA ABSTRACTION
Using a predefined data-abstraction tool. Extracted data were verified by a fourth investigator (CMW).10 was considered to represent significant heterogeneity between the trials. Patients with elevated SBP may take garlic to lower blood pressure. For each study. review of references in each obtained study) was used to identify additional relevant studies. we calculated a pooled variance for net change by standard methods. Volume 42
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.6. respectively. Results were limited to randomized controlled trials in humans. The following information from each included trial was recorded: first author. As suggested by Follmann et al. Studies were excluded if they were not randomized. CINAHL. or high degree of heterogeneity. For crossover trials. moderate.
STUDY SELECTION
To be included in the analysis. version 2. treatment effects) was used in calculating the weighted mean difference and its 95% confidence interval. age and sex of the study population. and the washout period (if it was a crossover trial). studies must have been written in English or German and reported endpoints of SBP or DBP. these trials had low power to detect differences and showed blood pressure effects ranging from large reductions to small elevations. KMR) independently recorded data for each included trial. version 2. as well as in those without elevated SBP. When data were presented individually for different treatment groups within a study.com
Studies with patients exhibiting a mean baseline SBP of greater than 140 mm Hg (and thus representative of stage I hypertensive patients) were analyzed separately from those with a lower mean baseline SBP in order to distinguish garlic’s effects on hypertensive and nonhypertensive patients. between 25% and 75%.2). values were recorded as a range of the reported means. double-blind. and dose of garlic being studied. A manual search of the literature (ie. Methods
DATA SOURCES
SBP and DBP values were recorded along with measurements of variance. For parallel trials in which variances for paired differences were reported separately for each group. In total. and allicin. Any disagreements were resolved through discussion with a third investigator (KMR). type. The mean change in SBP and DBP parameters from baseline was treated as a continuous variable and the weighted mean difference was calculated as the difference between the mean in the garlic and placebo groups. brand.3 we assumed a correlation coefficient of 0. they were calculated from confidence intervals. or if they did not report a baseline SBP. Two investigators (CT. For parallel trials. When the variance for paired differences was not reported. 3 investigators (CT. or placebo-controlled. the baseline and last reported
www. PV) independently reviewed all identified abstracts or full publications for inclusion and exclusion criteria. the level of heterogeneity was also assessed with I2 values where an I2 less than 25%. net changes were calculated as the mean difference in values at the end of the garlic and placebo periods. Cheshire. with disagreements resolved through discussion. in individuals without elevated SBP. net changes in each of these study parameters were calculated as the difference (garlic minus placebo) of the changes (baseline minus follow-up) in these mean values (also referred to as the change score). Since variances for net changes were not reported directly for most studies. did not report any measures of variance). Weighted mean differences were calculated using StatsDirect. Crossover studies were included as long as they provided a 2-week or longer washout period between treatments. p values. blood pressure reductions could be dangerous. Allium sativum...6. Publication bias was tested through visual inspection of funnel plots as well as the calculation of Egger’s weighted statistic (StatsDirect. The objective of our meta-analysis was to examine garlic’s effect on SBP and DBP in subjects with elevated SBP. if they lacked enough data for statistical pooling (eg. and greater than 75% would represent a minor. yet related. or individual variances for intervention and control groups/periods. Results Figure 1 displays the number of citations initially found upon conducting our search strategy and the number of studI
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DATA SYNTHESIS AND ANALYSIS
We conducted a systematic search of MEDLINE. We assumed equal variances during the trial and between intervention and placebo groups. while those without elevated SBP may take garlic for other reasons. All databases were searched from their inception through June 26. we calculated it from variances at baseline and at the end of follow-up. and the Cochrane Central Register of Controlled Trials to identify literature evaluating garlic’s effect on blood pressure. Statistical heterogeneity was assessed by calculating a Cochrane Q statistic whereby a p value less than 0.

respectively.2 to 26.3 mm Hg (95% CI 5.5) and DBP by 9. Statistical heterogeneity was minor. Forest plot results on garlic’s impact on SBP and DBP. and was not consid-
ered significant by means of the Cochrane Q statistic (p > 0. SBP = systolic blood pressure.3 mm Hg (95% CI 6. garlic reduced SBP by 16.com
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2008 December.7) were found in the group without elevated SBP.10 for all analyses).5 mm Hg in SBP (95% CI –2. as denoted by an I2 value less than 25% in all analyses. Publication bias was unlikely for the analyses in patients without elevated baseline SBP since the Egger’s statistic was greater than 0.3 to 13.1) and 0. In patients with elevated baseline SBP.12 The results of the meta-analysis are displayed in Figure 2. However. it could not be calculated in the analysis of subjects with elevated baseline SBP due to the small number of studies. reductions of only 0.3) compared with placebo.Effects of Garlic on Blood Pressure in Patients With and Without Systolic Hypertension: A Meta-Analysis
were not included. Discussion Garlic appears to significantly reduce SBP and DBP by 16 and 9 mm Hg. Volume 42
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.1 to 3.9 to 2. however.
www.9 mm Hg in DBP (95% CI –0.theannals. DBP = diastolic blood pressure. in patients with a mean
Figure 2.05.4 Only 1 study was known to be funded by a source other than the garlic manufacturer.

Fellow. Cutler J. 3.org
References
1. DOI 10. Evaluation. this was not seen in our meta-analysis.45: 769-73.com
. much more research is needed to better characterize its effects. our meta-analysis found that garlic selectively lowered SBP by 16 mm Hg and lowered DBP by 9 mm Hg. Health Outcomes. ours is limited by our use of aggregate data. Maulik SK. Storrs. an interaction between garlic and aspirin as well as other agents used to prevent thrombosis.
SBP greater than 140 mm Hg at baseline. While garlic can cause bad breath and body odor.
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2008 December. Nutr J 2002.18 we cannot adequately evaluate the risk of clinical adverse effects associated with chronic use. some subjects included in the elevated SBP
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analysis could have had normal or even low blood pressure and some subjects in the studies used for the nonelevated SBP analysis may have had high blood pressure. Cardiovascular Outcomes. Our findings are bound by several limitations. the effects of garlic on patients with diastolic hypertension alone were not evaluated. cmwhite@ harthosp. the level considered to be normal SBP. further. only the Kwai brand of garlic was used. this lack of outcomes data should preclude garlic’s use instead of standard antihypertensives such as thiazide diuretics. Overall. Because funding by the manufacturer of Kwai could not be ruled out for the 3 studies used in the elevated SBP analysis. 80 Seymour St. This is a pharmacologic phenomenon that has been reported with some antihypertensive medications: as blood pressure gets closer to normal. Chobanian AV. and Economics Collaborative Group. In other words. fax 860/545-2277. CT Payal Vachhani BSPharm. because it may not lower blood pressure in patients with normal blood pressure. University of Connecticut. Also. J Clin Epidemiol 1992. the actual impact of garlic on cardiovascular and cerebrovascular events is not known. and angiotensin-converting enzyme (ACE) inhibitors.1:4. the majority of trials in that analysis represented patients with prehypertension. Policy. such as the renoprotective effects of ACE inhibitors and does not have compelling indications for use. this is another reason not to use garlic in lieu of standard antihypertensive drugs for the treatment of hypertension. Future research focused on a larger number of hypertensive subjects may validate this finding and assess garlic’s long-term risk of harm. Health Outcomes. Beyond typical limitations of meta-analyses such as the inability to control the biases of included studies. School of Pharmacy. a review. Other brands of garlic require evaluation in hypertensive subjects. Effect of garlic on cardiovascular disorders. the risk of hypotension should be minimal. but other brands of garlic were used in 3 of the 7 studies in which patients had a lower mean baseline SBP. Bakris GL. Volume 42
www. we cannot recommend garlic specifically for its blood pressure–lowering effects in patients with prehypertension. calcium-channel blockers. independently funded studies with Kwai are also warranted. garlic did not lower blood pressure in nonhypertensive subjects. University of Connecticut and Hartford Hospital Reprints: Dr. Detection. PharmD Student. for example. It is possible that garlic administration has potentially serious adverse effects that were not reported in these studies. On the other hand.theannals. Black HR. therefore. and Treatment of High Blood Pressure. Follmann D. although the results of this meta-analysis show a promising future for garlic as an antihypertensive. School of Pharmacy. We grouped patients into categories based on mean baseline blood pressure. As such. Secondly. and there are a range of blood pressures that make up the mean. Seventh report of the Joint National Committee on Prevention. White.
Kurt M Reinhart PharmD. Variance imputation for overviews of clinical trials with continuous response. Garlic may have clinical utility in reducing total cholesterol and providing an antiplatelet effect1. Hartford Hospital. It is possible that there are inter. garlic products are not controlled by the Food and Drug Administration and therefore could theoretically have numerous constituents. Garlic did not impact blood pressure when we evaluated studies in which subjects’ mean baseline SBP was below 140 mm Hg. Methods Chief.and intrabrand differences in the concentrations of active constituents of garlic products. antihypertensives have an attenuated effect.. Also. et al. Director. For the time being. CT 06102. Hartford. Banerjee SK. This level of reduction is comparable to reductions seen with some prescription antihypertensive drugs and lends credence to the future exploration of garlic as an antihypertensive drug. Hypertension 2003. which could result in aggregation or ecological bias. Hartford. however. and Economics Collaborative Group. At this time. PharmD Student. Policy. University of Connecticut and Hartford Hospital Colleen Teevan BSPharm. University of Connecticut C Michael White PharmD.KM Reinhart et al. Conclusions In hypertensive patients. and contaminants that could have increased heterogeneity. unlike other antihypertensives. Elliott P.14 It is promising that observational studies not involving garlic found that blood pressure reductions in this range reduce the risk of coronary events and stroke by approximately 30% and 46%. University of Connecticut and Hartford Hospital. CT
Craig I Coleman PharmD. garlic is not known to have the beneficial effects outside of blood pressure reduction seen with several other antihypertensives. Larger-scale clinical trials are needed to allow an adequate sample size to be assessed for adverse effects.42:1206-52.15-17 However. In our analysis of studies with patients whose mean baseline SBP was greater than 140 mm Hg.14 In only 2 of the 7 studies were the average baseline SBPs at or below 120 mm Hg. Suh I. respectively.1186/1475-2891-1-4 2. their bioavailability.